The invention relates to a device comprising a first optical Mach-Zehnder interferometric sensor (MZI1) with a large FSR, wherein a plasmonic waveguide (107) thin-film or hybrid slot, is incorporated as transducer element planar integrated on Si3N4 photonic waveguides and a second optical interferometric Mach-Zehnder (MZI2), both comprising thermo-optic phase shifters (104, 106) for optimally biasing said MZI sensor (MZI1) and MZI as variable optical attenuator VOA. It further comprises an overall chip (112), being remarkable in that it comprises a set of Photonic waveguides (103) with a high index silicon nitride strip (303, 603), which is sandwiched between a low index oxide substrate (SiO2) and a low index oxide superstrate (LTO); Optical coupling structures (102, 109) at both ends of the sensor acting as the optical I/Os; an Optical splitter (102) and an optical combiner (109) for optical splitting at the first junction (102) of said first sensor (MZI1) and optical combining at the second junction (109) of said first MZI (MZI1); a variable optical attenuator (VOA) with said additional second MZI (MZI2), which is nested into said MZI1 (sensor)), deploying an optical splitter and an optical combiner for optical splitting at the first junction of said additional second MZI (MZI2), and optical combining at the second junction of said second MZI (MZI2); a set of Thermo-optic phase shifters (104, 106) to tune the phase of the optical signal in the reference arm (104, 106) of each said MZI (MZI1, MZI2-VOA); wherein Thermo-optic phase shifters are formed by depositing two metallic stripes parallel to each other on top of a section of the photonic waveguide and along the direction of propagation of light; and a plasmonic waveguide (107) in the upper branch (103) of said first MZI (MZI1), that confines light propagation through coupling to Surface Plasmon Polaritons (SPP) at the metal-analyte interface, and method associated thereto.